Sacking Mr Smith risks turning him into a martyr and establishing the principle that people can be banned from certain jobs purely because of their political views. I have minor experience of the latter. After September 11, 2001, I wrote an editorial stating that, if the US dropped bombs on other countries, it must expect attacks on its own territory and that, if they wished to be safer, Americans should elect a different government. This editorial caused outrage and I received a faxed letter from one influential American ( "a faxwa", I called it) which said he would campaign for my dismissal. He did not, he explained, question my personal right to hold and express such views. It was just that they should not be held by someone in my position.
This was how McCarthyism operated in 1950s America. The freedom of Communists to organise and publish was not denied. But they were hounded out of jobs where they could influence public opinion and the cultural climate, as Hollywood producers and actors could. Your right to freedom of speech and thought is severely limited if your political views prevent you from earning your living. So why would I still throw Mr Smith out of school?
Racial intolerance is more repugnant than political or religious intolerance because it allows its victims no respite. You can hide pro-capitalist or pro-Christian views and even dissemble commitment to Communism or Islam. Many people find this quite easy. But it is impossible to hide being black and hard to hide being Jewish.
This is why we still regard Hitler's Germany as worse than Stalin's Russia.
The latter was monstrous: it probably killed many more than did Nazi Germany and was itself viciously anti-Semitic. But anti-Semitism was not an article of faith. On the same principle, it was right to boycott apartheid South Africa but not necessarily other African regimes which used detention without trial, censorship and so on.
In these instances, there are no double standards, contrary to what is often claimed. To reserve special odium for Nazi Germany or apartheid South Africa is simply to say that racism is more odious than other odious political practices, rather as we say that raping a child is worse than raping an adult. Nor are there double standards in sacking a BNP teacher but not, say, a Communist who still defends Stalin's Russia. Even if the BNP teacher kept his views quiet in school, black children in his class might well be aware that he wanted them all to go "home", albeit "voluntarily". Or white children might think that, because the teacher held his job despite making his BNP membership public, racism and fascism were somehow acceptable. So Mr Smith will be a victim of intolerance - let us not doubt that - but of a lesser intolerance than he proposes for black people.
He could have sat at home thinking racist thoughts and chanting racist slogans while pretending to be a blameless liberal to the outside world. He could even have held BNP membership and kept it quiet. But blacks cannot keep quiet about being black or pretend to be white when they go out.
Peter Wilby is editor of the New Statesman